But Hersman made clear the severity of the incident as she addressed the grounding of the Dreamliners, while adding that the battery issues that prompted the grounding should have been prevented by the aircraft's design.

"This is an unprecedented event. We are very concerned," she told reporters at a news conference at the agency's Washington D.C. offices. "We do not expect to see fire events on an airplane. This is a very serious air safety event."

Despite multiple redundant safety features built into the system by Boeing, "those systems did not work as intended," Hersman said. "We need to understand why."

An NTSB team has disassembled and scanned the battery that caught fire. Investigators determined that internal short circuits had caused heating inside some of the battery cells. They found that the battery had suffered what's called a "thermal runaway," an uncontrolled overheating that spreads from cell to cell.

But they have not yet established the sequence of those events or whether either is the cause, or a symptom of what went wrong. Neither has the NTSB determined for sure whether the battery over-charged or if there could be internal manufacturing defects.

Boeing, which manufactures some parts for the Dreamliner at its operations in Salt Lake City, on Thursday said it "has formed teams consisting of hundreds of engineering and technical experts who are working around the clock with the sole focus of resolving the issue and returning the 787 fleet to flight status."

Tests ahead include a battery drawdown that will take a week. Hersman didn't have an answer for how long the investigation might take to come to a conclusion on the cause of the fire.

The Federal Aviation Administration, not the NTSB, must determine if the 787 will remain grounded. But given the lack of progress and the dire warning about how serious this event was, the FAA may not be in a position to lift the grounding any time soon.

The current Boeing 787 crisis began Jan. 7 when a small battery fire broke out aboard an empty Japan Airlines that half an hour earlier had landed at Boston's Logan International after a 12 hour flight from Tokyo.

All 183 passengers and 11 crew members had left the airplane when a mechanic making routine maintenance checks detected smoke in the cabin.

Airport firefighters found a battery fire in the rear electronics bay, in the lower fuselage just behind the wing. The jet was almost new, delivered to Japan Airlines only in December.

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